Drive systems utilizing pulleys or sprockets and flexible draft means such as chains or belts trained thereabout are well known. It is equally well known to provide for changes in the diameter of either or both the driving or driven pulley to accommodate changes in imposed load. In bicycles, the most commonly used variable speed/output drive requires a plurality of driving sprockets at both the crank and driven wheel. A shifting mechanism changes the chain from sprocket to sprocket. Although this system is in wide use, it does suffer from the disadvantages, inter alia, that (1) the shifting is not automatic but requires deliberate effort by the operator; (2) the chains often slip off the sprockets causing much inconvenience; and (3) the several speeds and power ratios available are fixed and may or may not be optimum for a given situation.
Various devices have been proposed to provide a load responsive system to automatically vary the driving sprocket diameter in response to load changes. One such arrangement, as described in U.S. Pat. No. 3,995,508 utilizes a driving sprocket assembly comprising a pair of spaced apart plates mounted on a hub. One plate is fixed to and rotates with the hub while the other is free to rotate about the hub axis. The fixed plate is provided with a plurality of spiral slots while the movable plate has an equal number of straight radially extending slots. A plurality of small sprockets are carried between the plates and each sprocket is provided with laterally extending elements, such as pins that are received in the slots. A spring co-acts between the plates urging them to the relative angular position that moves the sprockets farthest from the hub to define maximum effective sprocket diameter. In such prior arrangement, increased load forces the pins and sprockets to follow the slots toward the hub thus reducing sprocket diameter. In the arrangement described, the resulting chain slack is taken up by a separate take-up device. Although the device described appears generally to be workable, it suffers from the disadvantages that it is limited to a chain and is not adaptable to belts. Chains are undesirable due to initial as well as replacement cost, and also because they frequently malfunction due to failure to engage the sprockets precisely. Moreover, the range of speed and power ratio change is limited since the diameter of the small sprocket is fixed.
Systems have also been proposed utilizing belts which are desirable because they are relatively inexpensive, require little or no maintenance and, even more importantly, are not required to engage sprocket teeth hence will not jump off the pulley, but will maintain continuous contact. An early belt driven system, as described in U.S. Pat. No. 672,962, employs a pulley having a pair of spaced-apart plates fixed to a hub and at least one other plate rotatable relative to the hub and fixed plate. There is a plurality of spirally curved slots in the fixed plate and an equal number of straight radial slots in the movable plate. Arcuate belt-engaging segments between the plates have laterally extending guides that are received in the slots and mechanical means rotate the plates manually to move the segments and vary the pulley diameter. This system is not automatic. Also, the belt-to-pulley contact is erratic because the arcuate segments tend to roll in the direction of belt travel. Additionally, the rolling or tilting of the segments causes the segment guides to bind in the straight radial slots thus seriously hampering operation of the entire drive.
Another proposal, as disclosed in British Pat. No. 159,790, utilizes a pulley assembly that has a fixed and a movable plate mounted concentric to a hub. Each plate is provided with a plurality of pairs of parallel spirally extending curved slots. The curves of the two plates are identical, but extend in opposite directions. Arcuate belt-engaging segments are positioned between the plates, each segment being provided with pins that are received in the slots in both plates. The pulley diameters cannot vary automatically. Instead, complex means are provided to manually adjust the relative angular positions of the plates to effect radial movement of the segments. The structure requires parallel slots which cannot guide the segments radially except with pronounced tilting or rolling which reduces belt-to-pulley contact.
In summary, of the several prior arrangements described above, only the manually shiftable multiple sprocket chain drive has enjoyed any success. The load responsive chain and sprocket drive disclosed in patent 3,995,508 apparently suffers from the disadvantages of frequent misengagement of sprocket and chain while the prior belt and pulley arrangements do not and cannot provide sufficient and reliable belt-to-pulley contact throughout their range of operations for acceptable operation.